What makes a Chihuahua different from a Great Dane? A Jack Russell Terrier different from a Labrador Retriever? One PBGV different from another? In one word: genes.

Genes influence practically every aspect of a dog’s being. Some characteristics, like color, are controlled almost exclusively by the dog’s genes. For other more complex traits such as size and weight, genes share their influence with outside, environmental factors.

The basics

Each gene contains a set of instructions that tell part of the body how to grow and function. If all genes always contained the same instructions, then all dogs would be the same. While most genes are always the same, others can carry one of several different sets of instructions. Each possible set of instructions for a single gene is called an allele of that gene.

For example, one of the genes that govern coat color is called the B gene. This gene is believed to have two alleles, B and b. The B allele produces a black coat, while the b allele produces chocolate or liver coats.

Genes are organized in larger structures called chromosomes. The dog has 78 chromosomes or, more accurately, 39 pairs of chromosomes. Both chromosomes in a pair carry the same genes but not necessarily the same alleles of those genes. There is one exception to this rule: the so-called sex chromosomes. While a female carries a pair of X chromosomes, a male carries a single X chromosome and a single Y chromosome. The other 38 pairs of chromosomes are called autosomes.

How genes are passed from parents to offspring

Normal dog cells contain a full complement of 78 chromosomes. A male dog’s sperm and a female dog’s eggs, however, are a special kind of cell, called gamete, and only have 39 chromosomes.

During fertilization, the 39 chromosomes from the sperm combine with the 39 chromosomes from the egg to recreate a complete set of genetic instructions. In this way, the embryo receives exactly 50% of its genes from the sire, and 50% from the dam.(1)

When studying pedigrees, breeders often make the mistake of extending this 50/50 rule beyond the parents. They assume, for example, that 25% of a dog’s genes must have come from each of its four grandparents. Unfortunately, this isn’t so.

When each sperm or egg is formed in the parent, one chromosome is selected at random from each of the 39 pairs. There is no way to know whether the chromosome selected was the one originally contributed to the parent by the grand-sire or by the grand-dam. Moreover, through a process called crossing-over or recombination, the chromosome can be formed by combining parts of both.

It is, theoretically at least, possible for a gamete to contain only genes from one grandparent and none from the other. Practically, there will always be a mixture of both but the split is unlikely to be exactly 50/50.

Mendelian genetics: dominants and recessives

Have you ever tried to identify what ingredients went into a recipe by tasting the finished product? It’s not always possible. So it is with dogs: you can’t always tell what genes went in just by looking at the dog.

A dog’s outward, visible or measurable characteristics are collectively called its phenotype. This includes everything from the dog’s height and coat color, to the amount of thyroid hormone it produces. The dog’s genetic makeup, on the other hand, is called its genotype. Sometimes you can infer the genotype from the phenotype, and sometimes not.

Recall that, except for the genes carried on the sex chromosomes, a dog has two copies of every gene: one received from its sire and one received from its dam. If the gene has more than one allele, the dog might carry two identical alleles, or it might carry two different alleles. In the first instance, the dog is said to be homozygous for that allele; in the second, the dog is heterozygous.

Terms like dominant and recessive describe a hierarchical relationship between two alleles of the same gene. When an allele is dominant over another allele and both are present in the genotype, the phenotype only shows the effect of the dominant allele. The second allele is said to be recessive to the first, and its presence can only be observed in a homozygous individual.

For example, it is believed that a wire coat is controlled by a single gene, with the allele for wire coat, Wh, dominant over the allele for non-wire coat, wh. The table below illustrates the result of various combinations of the two alleles.

Since Wh is dominant, the phenotype is the same whether there are one or two copies of the Wh allele. It is impossible to find out by observation whether a wire-coated dog is homozygous for Wh or heterozygous.

If you do not want wire coats, they are easy to avoid because any individual carrying a Wh allele will be immediately identifiable and can be excluded from breeding.

On the other hand, if you are trying to produce dogs with wire coats, you might unknowingly breed two heterozygotes together and in that event are quite likely to produce some non-wire coated pups. Of course, when this happens you would immediately know that both parents carry the wh recessive and could adjust your breeding program accordingly.

As illustrated by this example, it is relatively easy to breed for desirable recessive alleles and to avoid undesirable dominant alleles. On the other hand, desirable dominant alleles can always mask undesirable recessive alleles, and these recessives can match up much later in your breeding program, seemingly “out of the blue,” and produce undesirable characteristics.

Quantitative traits: polygenic inheritance

What I have described so far explains the inheritance of traits that are controlled by a single gene, called a major gene. Such traits only have a few clearly distinguishable states. For example, the ticking gene T either produces ticking in the coat, or no ticking.

There are many other traits that do not fit this pattern: instead, they represent values that can be counted or measured on a scale. Examples of such quantitative traits are wither height, body weight, joint laxity, and litter size.

Quantitative traits are not controlled by major genes, but by several minor genes, also called polygenes, that are inherited independently of each other but work collectively. Some polygenes may work to increase the value of the trait, and others may work to decrease it, but the phenotype only reflects the “sum” of their effects. Often, quantitative traits are also affected by environmental factors.

Sometimes, polygenes act as modifiers to major genes. For example, various alleles of the S major gene control how much, if any, white spotting will be present on the coat. But dogs with the same S genotype can vary greatly in the amount of white on their coat: this is accounted for by modifier polygenes.

Genetically, minor genes work in the same way as major genes: they can have multiple alleles, and some alleles will be dominant over others. However, since we can’t observe the effect of each individual minor gene, we don’t talk about dominants and recessives.

Instead, we talk about the heritability of a quantitative trait, an indication of how well the trait can be passed on from parent to offspring. Heritability is calculated by statistical techniques and represents an average for the dogs studied. As they say, “your mileage may vary.”

The greater the heritability, the easier it is to change the trait by selecting appropriate breeding stock. For example, height at the withers has a relatively high heritability (40-65%.) If you select parents of above (or below) average height for the breed, they will tend to produce offspring that are above (or below) average in height.

On the other hand, traits with lower heritability are greatly influenced by environment and respond more poorly to selection. Litter size has a very low heritability (10-20%): there is only a small likelihood that daughters of dams that produce large litters will be better producers than the daughters of dams that produce small litters.
Conclusion

In this article, I have tried to provide a useful overview of a very complex subject. I hope it has sparked your interest and will encourage further study.

Acknowledgements

My thanks to Dr. Kasmin Bittle, DVM, for her advice, encouragement, and helpful comments during the writing of this article.